Her colleague, Senator Frank Drilon, who was also nominated, likewise declined the post.

"It's more fun in the Senate," Drilon said. "I feel I will be of better service to the country."

Santiago and Drilon will end their terms in 2016.

But because Santiago will not be able to finish her term because she was appointed as a judge in the International Criminal Court.

Santiago, however, clarified that her Senate post cannot be declared vacant yet by the Commission on Elections because she can't assume her post at the ICC until the judge she is replacing wraps up all his cases. - report from Jing Castañeda, ABS-CBN News; ANC